1. Field of the Invention
The present invention relates to a light emitting display, and more particularly to a light emitting display using electroluminescence of organic materials.
2. Description of the Related Art
In general, an organic light emitting diode display electrically excites phosphorus organic components and generates an image by voltage-programming or current-programming an n×m matrix of organic light emitting cells. These organic light emitting cells have features similar to a diode and are called organic light emitting diodes (OLEDs).
The OLED includes an anode, an organic thin film, and a cathode layer. The organic thin film layer has a multi-layered structure including an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) in order to balance electrons and holes and to enhance efficiency of light emission. Further, the organic thin film separately includes an electron injection layer (EIL) and a hole injection layer (HIL).
Methods of driving the organic light emitting cells having the foregoing configuration include a passive matrix method and an active matrix method. In the passive matrix method, an anode and a cathode are formed crossing each other and a line is selected to drive the organic light emitting cells. The active matrix method, on the other hand, employs a MOSFET or a thin film transistor (TFT). In the active matrix method, a pixel electrode of indium tin oxide (ITO) is coupled to the TFT, and a voltage that is maintained by a capacitor coupled to a gate of the TFT drives the light emitting cell. Depending on the type of signal transmission used for distinctively programming the voltage applied to the capacitor, the active matrix method is classified into a voltage programming method and a current programming method.
A pixel circuit of an organic light emitting display employing the active matrix method is described below. FIG. 1 shows a pixel circuit for a pixel located on a first row and a first column among the n×m matrix of pixels. A pixel 10 has three sub-pixels 10r, 10g, 10b which use OLEDs. Depending on the color of the light emitted by these diodes, they are labeled OLEDr, OLEDg, and OLEDb emitting lights of red R, green G, and blue B, respectively. The sub-pixels are arranged in a strip format with each of pixels coupled to a separate data line D1r, D1g, D1b and all of the pixels coupled to one common scan line S1.
The red sub-pixel 10r, that generates red light, includes a driving transistor M1r, a switching transistor M2r, and a capacitor C1r to drive the OLEDr. Similarly, a green sub-pixel 10g, that generates green light, includes a driving transistor M1g, a switching transistor M2g, and a capacitor C1g, and a blue sub-pixel 10b, that generates blue light, includes a driving transistor M1b, a switching transistor M2b, and a capacitor C1b. 
All of the red, green, and blue sub-pixels 10r, 10g, 10b operate similarly. Therefore, the operation of the red sub-pixel 10r will be described as a representative example. The driving transistor M1r is coupled between a power source of voltage VDD and an anode of the OLEDr, and sends a current to the OLEDr for light emission. A cathode of the OLEDr is coupled to a voltage VSS which is lower than the power source voltage VDD. The amount of current of the driving transistor M1r is controlled by a data voltage applied through the switching transistor M2r. The capacitor C1r is coupled between a source and a gate of the driving transistor M1r and maintains a voltage applied between the source and the gate of the driving transistor M1r for a predetermined period of time. A scan line S1 transmitting an on/off selection signal is coupled to a gate of the switching transistor M2r, and a data line D1r transmitting a data voltage corresponding to the red sub-pixel 10r is coupled to a source of the switching transistor M2r. 
When the switching transistor M2r is turned on in response to the selection signal applied to the gate of the switching transistor M2r, a data voltage VDATA is applied to the gate of the driving transistor M1r through the data line D1r. Consequently, a current IOLED corresponding to a voltage VGS charged between the gate and source of the driving transistor M1r by the capacitor C1r flows through the driving transistor M1r, and the OLEDr emits light corresponding to the current IOLED. The current IOLED flowing to the OLEDr is given by Equation 1.
                                                                        I                OLED                            =                                                β                  2                                ⁢                                                      (                                                                  V                        GS                                            -                                              V                        TH                                                              )                                    2                                                                                                        =                                                β                  2                                ⁢                                                      (                                                                  V                        DD                                            -                                              V                        DATA                                            -                                                                                                V                          TH                                                                                                              )                                    2                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            where, β is a constant number representing the gain of the transistor M1r and VTH is the threshold voltage of this transistor.
As seen in Equation 1, the current IOLED applied to the OLEDr, that is proportional to the data voltage VDATA, causes the OLEDr to emit light with luminance corresponding to the current IOLED. The applied data voltage VDATA is maintained within a predetermined range in order to express brightness according to predetermined scales.
As described above, in the organic light emitting display, a pixel 10 has three sub-pixels of red, green, and blue 10r, 10g, 10b and each of the sub-pixels has a driving transistor M1r, M1g, M1b, a switching transistor M2r, M2g, M2b, and a capacitor C1r, C1g, C1b to drive a corresponding OLEDr, OLEDg, OLEDb. Further, each of the sub-pixels 10r, 10g, 10b includes a data line D1r, D1g, D1b to transmit a data signal and a power line to transmit a voltage VDD. Many wires are required to drive a pixel 10, causing difficulty in arranging the wires within a pixel area and decreasing an opening ratio available for actual display. Thus, development of a pixel circuit requiring less wiring and fewer elements for driving a pixel is desirable.